The issue of contrasts of colors of an object when such object is illuminated with light is known in the art. US2009122530, for instance, describes solid state illumination systems which provide improved color quality and/or color contrast. The systems provide total light having delta chroma values for each of the fifteen color samples of the color quality scale that are preselected to provide enhanced color contrast relative to an incandescent or blackbody light source, in accordance with specified values which depend on color temperature. Illumination systems provided in US2009122530 comprise one or more organic electroluminescent elements, or these comprise a plurality of inorganic light emitting diodes, wherein at least two inorganic light emitting diodes have different color emission bands.
WO2005/115059 describes a white light emitting element having a spectrum in a wide wavelength range. WO2005/115059 desires to provide a white light emitting element in which chromaticity of white color is hard to change over time. Further, WO2005/115059 desires to provide a white light emitting element in which the shape of an emission spectrum does not tend to depend on current density. A first light emitting element and a second light emitting element are serially laminated over a substrate. The first light emitting element has a light emitting layer between a first anode and a first cathode, and the second light emitting element has a light emitting layer between a second anode and a second cathode. The light emitting layer shows a first emission spectrum having peaks both in a blue to blue green wavelength range and in a yellow to orange wavelength range, and the light emitting layer shows a second emission spectrum having peaks both in a blue green to green wavelength range and in an orange to red wavelength range.
This document provides an example of a light emitting element in which a first light emitting element and a second light emitting element are laminated in series over a substrate. The second light emitting element has a light emitting layer between a second anode and a second cathode. The light emitting layer of the first light emitting element includes a first light emitting layer showing an emission spectrum having a peak in a blue to blue green wavelength range and a second light emitting layer showing an emission spectrum having a peak in a yellow to orange wavelength range. The light emitting layer of the second light emitting element includes a third light emitting layer showing an emission spectrum having a peak in a blue green to green wavelength range and a fourth light emitting layer showing an emission spectrum having a peak in an orange to red wavelength range. Note that the order of lamination of the first light emitting layer and the second light emitting layer may be the reverse. The order of lamination of the third light emitting layer and the fourth light emitting layer may be the reverse. When a positive bias is applied to the first anode side and a negative bias is applied to the second cathode side of the light emitting element, first light and second light can be provided. The first light is a combination of both light emitted from the first light emitting layer and the second light emitting layer; thus, it shows an emission spectrum having peaks both in a blue to blue green wavelength range and in a yellow to orange wavelength range. In other words, the first light is dual wavelength white or near-white emission color. The second light is a combination of both light emitted from the third light emitting layer and the fourth light emitting layer; thus, it shows an emission spectrum having peaks both in a blue green to green wavelength range and in an orange to red wavelength range. In other words, the second light emitting element shows dual wavelength white or near-white emission color which is different from that of the first light emitting element. Thus, the light emitting element of the invention can provide light which covers a blue to blue green wavelength range, a blue green to green wavelength range, a yellow to orange wavelength range, and an orange to red wavelength range as a result of overlapping the first light with the second light. The first light emitting element and the second light emitting element each have a similar structure to that of a dual wavelength white light emitting element using a complementary color relationship which is often conventionally used, and can realize a white or near-white light emitting element having high luminance and favorable element life. However, the first light emitting element shows a poor spectrum mainly in a blue green to green (specifically, emerald green) wavelength range and in an orange to red wavelength range, and is not suitable for a full color display using a color filter. In addition, the first light emitting element has a narrow spectrum in an emerald green wavelength range and lacks in vividness.